1. Field of the Invention
The present invention relates to a printing control method for a printer, wherein the printing speed is controlled depending on the printing contents to be printed by a printing head.
2. Description of the Prior Art
In an ordinary printer, a printing head is placed to face printing paper on a platen, and is driven in accordance with printing data while a spacing motor is controlling the spacing action of this printing head in the primary scanning direction of the printing paper. Here, spacing means the operation in which the printing head is moved in the direction perpendicular to the direction of printing paper delivery.
In the case of a wire-dot type printing head, since many wires must be electrically driven, a large amount of electrical power is needed. Also, in the case of a printer which uses a thermal head with a large number of thermal elements, a lot of electrical power is needed, depending on the printing data. At the same time, a spacing motor, which spaces the printing head in the primary scanning direction, also consumes a large amount of electrical power. A single electrical power source is used to drive both the thermal head and spacing motor.
However, recently, there have been strong demands to make the printer small and light, less expensive, multi-functional, and less power consumptive, which translates into strong demands for minimizing the capacity of the electrical power source.
If the spacing speed of the printing head is increased, proportionally more power is consumed. Therefore, it is feared that if it is desired to perform high density printing at high speed, the power source capacity may be exceeded. That is, if graphic characters and the like are printed during high speed printing, the power supplied to the printing head becomes insufficient, reducing the printing density.
In the past, various methods were adopted in order to solve such a problem. For example, the spacing speed was reduced in the case of graphic printing in which printing dot density is high compared to printing of ASCII characters.
Further, if such a control is performed for each unit line, the overall printing speed slows down even for the line containing only one graphic character since the printing speed slows down for this character. Therefore, a device had been introduced (Japanese Patent Publication No. 42632/1986), in which the printing speed was different between Chinese characters and others and the printing head was controlled to reduce the printing speed if the count of Chinese characters mixed in each line of printing exceeded a prescribed value.
Further, another device, explained below, had been developed for the same purpose (Japanese Patent Publication No. 19348/1989). FIG. 2 shows a block diagram showing the essential section of this prior printer. In the device, printing action is controlled by control section 1. This device is provided with head driving circuit 2, printing head 3, motor driving circuit 4, spacing motor 5, analog/digital conversion circuit 6, logic circuit power source 7, power source 8, voltage detection circuit 9, and digital/analog conversion circuit 10.
Head driving circuit 2 is a circuit for controlling the printing action of printing head 3, and operates by receiving its power from power source 8. Motor driving circuit 4 is a circuit for controlling spacing motor 5, and supplies spacing motor 5 with pulses proportional to a necessary spacing speed if spacing motor 5 is a pulse motor. This motor driving circuit 4 is also supplied with power by power source 8. Logic circuit power source 7 is a power source for supplying control section 1 with operational power.
Furthermore, control section 1 supplies head driving circuit 2 with printing data, supplies motor driving circuit 4 with a control voltage through digital/analog conversion circuit 10, and controls the spacing so as to be performed at a set speed. Incidentally, motor driving circuit 4 and analog/digital conversion circuit 6 and digital/analog conversion circuit 10 constitute a servo system for spacing motor 5 and control the spacing so as to be performed at a set speed.
More specifically, motor driving circuit 4 supplies spacing motor 5 with control power proportional to the set speed, and spacing motor 5 rotates at the set speed. Spacing motor 5 is provided with a rotation detection sensor, not illustrated, such as a rotary encoder, and the output from this sensor is sent to analog/digital conversion section 6, digitalized, and inputted to control section 1. Control section 1 recognizes the actual speed of spacing motor 5 and increases or decreases the control voltage outputted to digital/analog conversion section 10, so that the spacing is performed at the set speed.
Here, this device is configured so that voltage detection circuit 9 detects the output voltage of power source 8, sends the output to the above mentioned servo system in order to control variably the spacing speed of spacing motor 5.
Specifically, power source 8 simultaneously supplies head driving circuit 2 and motor driving circuit 4 with their driving power. Here, in the case of a printing contents such as graphic data or Chinese characters, wherein printing head 3 consumes a large amount of power to print, the source voltage of power source 8 drops, as motor driving circuit 4 tries to drive spacing motor 5 at a high speed.
Voltage detection circuit 9 detects this voltage drop and sends the above mentioned servo system a command to slow down the spacing speed. In this servo system, when digital signals proportional to the spacing speed are sent out from control section 1, they are converted to a corresponding analog voltage in digital/analog conversion circuit 10, and motor driving circuit 4 is controlled by this analog voltage.
Voltage detection circuit 9 adjusts the control voltage outputted from digital/analog conversion circuit 10 based on the printing contents. For example, it controls the spacing speed to be faster in the case of ASCII characters and slower in the case of graphic characters.
According to such arrangement, since the spacing speed is adjusted when power voltage actually drops, the total printer throughput can be increased in comparison to the case in which the spacing speed is uniformly set for each unit line.
Further, in the case of a printer with a wire-dot type printing head, large noises are generated while printing is going on, since many wires are driven. Because this noise level has a positive correlation with the number of impacts per unit time, if the printing speed is increased, the noise level gets higher accordingly. Therefore, in order to lower the noise level, the printing speed had better be decreased, but the operating speed of a printer is slow compared to other I/O devices, and demands for improving the operating speed, in other words, the printing speed, are strong. Because of this reason, it is necessary to increase the printing speed as much as possible, which brings out results that are contrary to noise level reduction.
In order to handle this contradiction, there was a method in which a normal printing mode and a low noise level printing mode were provided so as to offer a selection during printing. Most of the noises from a printer belong to the noises generated during actual printing actions, especially those generated by printing head 3, and this printing sound is positively correlated to the number of impacts per unit time. Therefore, in order to reduce the number of impacts per unit time, printing was done at a lower printing speed in the low noise printing mode, rather than in the normal printing mode.
Further, this low noise printing mode was set by a control command from a host computer, not illustrated, or by an input through a switch on an operating console panel, not illustrated, and the switching was made, by control section 1, from the mode with a normal printing speed to the mode with a low noise printing speed, which is slower than the normal printing speed, and then the printing continued.
However, if the spacing speed is variably controlled corresponding to the printing contents as mentioned above, spacing motor 5 is accelerated or decelerated while proceeding in the primary scanning direction.
Generally speaking, the relationship between the spacing interval and the spacing speed during acceleration or deceleration can be expressed by following equation. EQU X=(V.sup.2 -V0.sup.2)2.alpha. (1)
where X represents a spacing interval, V a spacing speed, VO an initial speed of the printing head, and .alpha. represents an acceleration. As is evident from this equation, with acceleration .alpha. being kept constant, the moving distance X from the beginning of acceleration to the time when a target speed is achieved becomes quadrupled if the target speed is doubled.
FIG. 3 shows an explanatory diagram showing the operation of the printer shown in FIG. 2. FIG. 3(a) is a graph in which power source voltage VP is plotted on the vertical axis and printing location (spacing interval) X on the horizontal axis. FIG. 3(b) is a graph in which printing speed (spacing speed) V is plotted on the vertical axis and printing location X on the horizontal axis. Furthermore, FIG. 3(c) is a graph in which the number of printing dots n is plotted on the vertical axis, and printing location X on the horizontal axis.
These graphs show the changes of power source voltage corresponding to the number of printing dots when the initial output voltage VP of power source 8 is VP0 and the printing speed is V0, and in addition, show the changes of the same power source voltage when the printing speed is increased to twice V0 in order to increase the printing speed.
Incidentally, the printing pattern is set, as is shown in FIG. 3(c), so that the printing head travels left to right from location X1 to location X4 without taking printing actions until it reaches location X2, prints in 24 dot format from location X2 to location X3, and does not print beyond that point.
Here, with the printing speed being V0, the printing head prints in 24 dot format at printing location X2, and meanwhile, the power supply from power source 8 to head driving circuit 2 increases. At this time, if the output voltage is detected by voltage detection circuit 9, the speed of spacing motor 5 is decreased at printing location X5 and is stabilized to V1 at printing location X7. Incidentally, the power source voltage drops to VP1 in this case. When the printing head passes printing location X3 where printing in 24 dot format ends, power source voltage recovers to VP0 and spacing motor 5 is accelerated again to V0.
On the other hand, in the case of printing speed 2V0, when 24 dot printing begins at printing location X2, power source voltage VP0 suddenly drops, and in response to this drop, the printing speed is reduced to V1. However, because the initial printing speed is fast, the printing speed finally slows down to V1 when the printing head arrives at printing location X8. Past this point, printing goes on at this constant speed of V1 until the printing head arrives at printing location X3 where the number of printing dots returns to "0," and past printing location X3, the printing speed is accelerated back to the initial printing speed of 2V0.
It is the interval [1] in FIG. 3(a) where a problem occurs. In this interval, printing head 3 must perform printing at printing speed V1, at which a proper balance is maintained between the power for printing and the power for driving the spacing motor, but printing continues without changing speed from the fast speed.
The relationship expressed by the above Equation (1) equally holds true for both acceleration and deceleration cases, and power source 8 outputs the same amount of power to motor driving circuit 4 for the deceleration of spacing motor 5 as for the acceleration. Therefore, power source 8 is required to output more power than its capacity in interval [1] in FIG. 3(a) and the power to be supplied to printing head 3 becomes insufficient. As a result, in the case of a wire-dot type head, for example, printing errors such as faint prints or missing dots occur. Needless to say, the same phenomena also occur during acceleration.
As a matter of fact, if a motor with better acceleration and deceleration response or a power source with a larger capacity is used, problems such as the above do not occur. However, a motor with better acceleration and deceleration response becomes large, and increasing the power source capacity results in an increased size of the power source itself. Therefore, the problems could not be solved by such a method as described above since there is a demand to reduce the size and costs of printers.
Furthermore, the printing control device of the prior printer controlled the printer so as to operate in the low noise printing mode regardless of the printing contents in order to reduce the number of impacts per unit of time. However, since such a control method made the printing speed uniformly slow however or not a printing pattern had a low printing dot density, and therefore had a relatively low noise level, there was a problem that the total throughput for the printer decreased.
Furthermore, as an attempt to obtain a low noise printer without reducing the throughput, the installation of sound proofing or sound shielding structure was practiced, but in this case, there was a problem that the addition of sound proofing or sound shielding structures led to cost increases.